1. Field of the Invention
The present invention relates to a reactor blend polypropylene suitable as a thermoplastic-elastomeric material and to a process for the preparation thereof. It also relates to a process for the preparation of indenyl metallocene ligands.
2. Description of the Prior Art
As it is well known, the polymers of propylene can be either crystalline or amorphous. Whereas the polypropylene having isotactic or syndiotactic structure is crystalline, the polypropylene having essentially atactic structure appears to be amorphous. The atactic polypropylene, represented by the Fischer formula as described in "M. Farina, Topics Stereochem., 17, (1987), 1-111" shows methyl groups casually arranged on each side of the polymeric chain. As described in the above mentioned publication, useful information on the structure can be obtained from the N.M.R. analysis.
While crystalline polypropylenes are widely used in the manufacture of plastic products, the amorphous polypropylene available on the market, which is generally a by-product of the isotactic polypropylene obtained with Ziegler-Natta type catalysts, is mainly used in adhesive compositions and as additives for bitumens.
Polymers of propylene are further known having low crystallinity degree and showing elastomeric characteristics.
U.S. Pat. No. 4,335,225 discloses a fractionable, elastic polypropylene, having an isotactic content of 55% or less, which contain a diethyl ether-soluble fraction with an isotactic crystalline content of about 0.5-5% by weight. This polypropylene is prepared with a catalyst based on tetraalkyl zirconium supported on a metal oxide.
U.S. Pat. No. 4,971,936, U.S. Pat. No.5,089,573, U.S. Pat. No. 5,118,649, U.S. Pat. No. 5,118,767, U.S. Pat. No. 5,118,768 and U.S. Pat. No. 5,294,581 disclose elastomeric, primarily isotactic polymers of propylene, having a narrow distribution of relatively short block lenghts.
U.S. Pat. No. 5,270,276 and U.S. Pat. No. 5,270,410 disclose elastomeric, primarily syndiotactic polymers of propylene, having a narrow distribution of relatively short block lenghts.
However, all the above elastomeric polypropylenes, due to the fact that the catalyst systems which are used for their preparation have different catalytic sites, are endowed with a wide distribution of molecular weights which reflects negatively on their properties.
More recently, catalysts based on metallocene and alumoxane compounds have been used in the polymerization reaction of olefins. Operating in the presence of these catalysts, polymers characterised by a narrow molecular weight distribution and having structural characteristics of interest have been obtained.
By polymerizing propylene in the presence of metallocene catalysts, amorphous or highly crystalline polypropylenes can be obtained depending on the metallocene used.
In particular, chiral, bridged metallocene compounds give rise to stereospecific catalysts able to polymerize propylene to highly crystalline polymers. European patent application EP 185,918, for instance, discloses a process for the preparation of isotactic polypropylene in the presence of the ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride/alumoxane catalyst system.
Polymers of propylene are further known, obtainable in the presence of particular bridged metallocene catalysts, which have isotactic and atactic blocks within a single polymer chain and exhibits elastomeric properties. Polymers of this type are described by Llinas et al. in "Macromolecules, 1992, 25, 1242-1253". The polymerization is carried out in liquid toluene. The characteristics of the obtained polymers depend on the temperature of polymerization: the polymers obtained at 25-50.degree. C. are completely soluble in diethyl ether and shows a very low melting point of about 70.degree. C., while the polymers obtained at .ltoreq.0.degree. C. have diethyl ether insoluble fractions but exhibit no melting endotherm. Despite the homogeneity in molecular weight distribution, the molecular weights of these polymers are not high enough and their poor crystallinity makes them unsuitable for certain applications, such as for compatibilizing blends of amorphous and crystalline polyolefins.
The problem which the present invention sets out to solve is, therefore, to provide partially crystalline thermoplastic-elastomeric propylene polymers, directly obtainable from the polymerization reaction of propylene without the need of separation steps or of sequential polymerization, which are endowed with good mechanical properties and can be suitably used as an elastomeric material and as a compatibilizer for blend s of amorphous and crystalline polyolefins.